Crack opening displacement(COD) was applied to characterize the fracture initiation of the tough high density polyethylene, for the concepts of linear fracture mechanics(LEFM) are inapplicable for reasonably sized specimens due to extensive plasticity. In this paper normal single side notched three-point bend specimens and silica rubber replica techniques were used to study the characteristic COD of high-density polyethylene pipe and its butt-fusion joints including the weld fusion zone and heat effected zone at different temperature from -78℃ to 20℃. Testing results show that characteristic COD appears to depend on structural features that are determined by welding process and on testing temperature. As the temperature is lowered, characteristic COD of every zone studied decreases. Because welding process significantly changes some structural feature of HDPE material, characteristic COD of weld fusion zone is the smallest among three zones. Testing results can be used for the engineering design and failure analysis of HDPE pipe.